1. Field of the Invention
The present invention relates to a control system and control method for controlling a heater that heats and activates an oxygen concentration detector provided in an exhaust passage of an internal combustion engine when the engine is started, as well as an engine control unit.
2. Description of the Related Art
Conventionally, a control system of the above-mentioned kind has been disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. H11-218044. In the control system, during the start of an internal combustion engine, a heater for heating an oxygen concentration sensor as an oxygen concentration detector is duty-controlled in two stages as described hereinbelow. More specifically, the duty ratio of a control signal to be inputted to the heater is held at a maximum value (100%) until a predetermined time period has elapsed after turn-on of an ignition switch (hereinafter referred to as “the IG·SW”). The reason for this is that the oxygen concentration sensor is required to be heated up to an activation temperature as quickly as possible so as to prevent exhaust emissions from being increased since proper air-fuel ratio control based on a detection signal from the oxygen concentration sensor cannot be executed before full activation of the oxygen concentration sensor.
Then, after the lapse of the predetermined time period after the turn-on of the IG·SW, the duty ratio of the control signal to be delivered to the heater is determined through calculation of a basic value by retrieval from a map according to the rotational speed of the engine (hereafter referred to as “the engine speed”) and load on the engine, and correction of the basic value by a correction coefficient retrieved from a table according to a time period elapsed after the turn-on of the IG·SW. A correction executing time period over which the above-mentioned correction is performed, i.e. part of an energization time period for energizing the heater, over which the above-mentioned correction is performed, is set by searching a table according to the engine coolant temperature of the engine detected by an engine coolant temperature sensor when the IG·SW is turned on. The correction coefficient and the correction executing time period are thus set by estimating the temperature of the oxygen concentration sensor at the time when the IG·SW is turned on, and setting the duty ratio of the control signal and the energization time period for the heater according to the estimated temperature, so as to prevent the service life of the oxygen concentration sensor from becoming short due to overheating thereof by the heater.
In the conventional control system described above, the duty ratio of the control signal to the heater is controlled only in two stages, and the heater is controlled to the maximum duty ratio, regardless of the actual temperature of the oxygen concentration sensor, until the predetermined time period elapses after the turn-on of the IG·SW. Therefore, the control accuracy is low. For example, even when a stoppage time period of the engine before the present start was short and the oxygen concentration sensor has already reached the activation temperature, the heater is controlled to the maximum duty ratio, so that battery power is wastefully used and the service life of the sensor can be reduced due to overheat. Further, the oxygen concentration sensor is disposed at a location remote from the engine coolant temperature sensor that detects the coolant temperature of the engine, and the temperature-related properties, such as specific heat, of the oxygen concentration sensor are different from those of coolant for the engine, so that after turn-off of the IG·SW, the actual temperature of the oxygen concentration sensor changes differently from the engine coolant temperature. For this reason, the engine coolant temperature at the restart of the engine after the turn-off of the IG·SW does not accurately reflect the actual state of the temperature of the oxygen concentration sensor. Therefore, if the heater is controlled according to the coolant temperature, the control accuracy becomes low. For example, after the restart of the engine, the oxygen concentration sensor whose temperature has already reached the activation temperature, as in the above-mentioned case, can be overheated by the heater.